The present invention relates to a catalytic converter for the exhaust system of an automotive vehicle, and more particularly relates to a catalytic converter for the exhaust system of an automotive vehicle, which is of the so called monolithic catalyst type, and which is provided with a means for coping with variations in the operating temperature of the parts thereof.
Catalytic converters are in wide use nowadays for purifying the exhaust gases from internal combustion engines of automotive vehicles. When exhaust gases are passed through such a catalytic converter they may be purified of HC and CO and unburnt hydrocarbons contained therein, as well as being purified of nitrogen oxides (generically referred to as NOx) which they contain. A common type of such a catalytic converter is the so called monolithic type catalytic converter, which incorporates a so called monolithic catalyst body. This monolithic catalyst body is made of an integral mass of ceramic material, which used as a catalyst carrier, and which has a honeycomb structure which has a very large surface area. Catalytic metal is carried by this mass of ceramic material as laid in a thin layer on its said very large surface area. Ceramic material is used for the carrier because it is suitable for being formed into the requisite finely detailed shape and because it is capable of withstanding the high temperatures associated with such catalytic action, and because further it does not disturb the catalytic action of the catalyst metal laid thereon; but the difficulty has arisen that such a ceramic material is rather brittle and fragile, and accordingly the mounting of such a monolithic catalyst body within the tubular casing or body of the monolithic catalytic converter, in such a way that the monolithic catalyst body is protected from vibration and shock, such as for example vibration and shock caused by the operation of the internal combustion engine, so that it can be expected to have a long service life, is difficult. Further, the difficulty of mounting the monolithic catalyst body within the tubular casing of the monolithic catalytic converter is greatly aggravated by the fact that the thermal expansion coefficients of the ceramic material of the monolithic catalyst body and of the metal which typically forms said tubular casing of the monolithic catalytic converter are very different, and accordingly under the high temperatures characteristic of the catalytic conversion process performed by the monolithic catalyst body, which can easily exceed 800.degree. C., the differential expansion of said ceramic material of the monolithic catalyst body and of the metal of said tubular casing of the monolithic catalytic converter is very considerable. The differential expansion can impose stresses on the monolithic catalyst body which are quite capable of fracturing its relatively brittle and fragile structure, unless adequate means are utilized for properly mounting the monolithic catalyst body within the tubular casing of the monolithic catalytic converter.
A method which has been adopted in the past of holding the monolithic catalyst body within the tubular casing of the monolithic catalytic converter has been for the monolithic catalyst body, which is of a generally columnar shape, to be held between two cushion rings at its two opposite ends, said cushion rings being manufactured of a relatively soft substance. Thus the monolithic catalyst body has been axially held with a certain amount of cushioning which has been sufficient to allow for the differential expansion of the monolithic catalyst body and the tubular casing of the monolithic catalytic converter within which it is held; and also these cushion rings protect the monolithic catalyst body from vibration and other shock, such as for example vibration and shock due to the operation of the internal combustion engine to which the monolithic catalytic converter is attached, to which it might be otherwise subjected. Further, the monolithic catalyst body has been held, with regard to its radial positioning within the tubular casing of the monolithic catalytic converter, by means including wire mesh and/or foamed thermally resistant material, said means being interposed between the outside surface of the monolithic catalyst body and the inside cylindrical surface of said tubular casing of the monolithic catalytic converter, and said radial clamping further protecting the monolithic catalyst body from vibration and other shock. Further, it has been conventionally known and practiced for these cushion rings to be engaged with retainer rings which hole them within the tubular casing of the monolithic catalytic converter. If these retainer rings are appropriately designed, they can prevent the cushion rings from coming into direct contact with the relatively hot exhaust gases which are being emitted by the internal combustion engine and are being flowed through the monolithic catalytic converter to purify them. This is important because the material of which the cushion rings are made, which needs to be relatively soft and needs to provide a certain cushioning function, is necessarily less resistant to the hot exhaust gases than the material of such retainer rings, which is typically steel, in particular stainless steel. The retainer rings also can perform the very important function of keeping the cushion rings in proper shape, preventing them from becoming shifted or distorted or from being displaced from their proper positioning as holding the columnar monolithic catalyst body. This again is important because the material of which the cushion rings are made is as stated above relatively soft and accordingly is more liable to distortion than the material of such retainer rings.
Now, typically, the interior periphery of such a retainer ring is directly exposed to the full temperature of the exhaust gases which are being emitted by the internal combustion engine and are being flowed through the monolithic catalytic converter to purify them, and the temperature of these exhaust gases can attain 800.degree. C. Accordingly, the interior periphery of such a retainer ring can attain a temperature of 600.degree. C. to 650.degree. C. On the other hand, the exterior periphery of such a retainer ring is typically in contact with or is very close to the inner surface of the tubular casing of the monolithic catalytic converter, and accordingly is kept fairly cool. Thus the inner periphery of the retainer ring is typically heated up to a much higher temperature than the outer periphery thereof. Further, since as stated above typically the exterior periphery of such a retainer ring is in contact with the relatively cool inner surface of the tubular casing of the monolithic catalytic converter, the radial expansion of the retainer ring is typically substantially prevented. Accordingly, the problem of differential heating of such a retainer ring becomes acute, and the retainer ring is liable to undergo thermal deformation such as waviness, during use of the monolithic catalytic converter. Such waviness or other deformation of the retainer ring has the detrimental effect that the retainer ring thereby becomes much less able properly to hold the cushion ring, and accordingly it is risked that the cushion ring should become displaced from its proper position, thereby causing the holding of the monolithic catalyst body by the cushion ring to fail, which can allow the monolithic catalyst body to thereafter become damaged by shock and/or vibration. Further, the deformation of the retainer ring can allow the impact of hot exhaust gases on the cushion ring or upon the means for radially positioning the monolithic catalyst body within the tubular casing of the monolithic catalytic converter such as the aforementioned wire mesh and/or foamed thermally resistant material interposed between the outside surface of the monolithic catalyst body and the inside cylindrical surface of said tubular casing of the monolithic catalytic converter, and this impact of hot exhaust gases can deteriorate said cushion ring or the material of said radial positioning means, thus again perhaps causing the holding of the monolithic catalyst body by the cushion ring to fail or allowing the monolithic catalyst body to become damaged by shock and/or vibration.
A further disadvantage of the above outlined prior art type of retainer ring for a monolithic catalytic converter is that such a retainer ring has been typically manufactured out of a costly material such as stainless steel, which is resistant to the hot exhaust gases over a long period of time, by such a process as stamping out of stainless steel plate and deburring, for example. Now, such a method of construction tends to waste at least 80% of the stainless steel plate material, which is very costly and troublesome, and is ecologically unsound.